Wafer manufacturing frequently requires accurate control of the wafer temperature. For example, during plasma applications such as deposition or etching, heat is removed from the wafer, typically by a heat transfer gas. Most commonly, the heat transfer gas employed is helium.
In one conventional approach the wafer is mechanically clamped from the periphery of its top surface and helium pressure is supplied from the bottom side of the wafer. This technique can result in wafer non-uniformities and bowing.
Other approaches rely on electrostatic wafer holding. Both coulombic and Johnson-Rahbek types of electrostatic chucks can be employed during processing to secure a wafer or another workpiece, such as, for example, flat panel displays, onto a chucking surface. Some of the available electrostatic chuck designs include an electrode and an insulating or semiconducting layer of material between the electrode and the chucking surface and through-holes for passing helium gas from the back surface to the chucking surface.
While electrostatic forces can be distributed uniformly over the entire chuck surface, thereby ameliorating wafer bowing, plasma can penetrate the helium through-holes and sometimes can extinguish itself. Furthermore, helium arcing can occur through the straight line path between the back surface of the chuck and the electrode.
In one available design a heat transfer fluid pathway is provided in a conductive chuck pedestal, typically fabricated from aluminum metal and overlaid by a dielectric layer, typically formed by spray-coating a ceramic material such as alumina or alumina/titania. The pathway can include a porous dielectric insert which can isolate the conductive pedestal from the pathway. While this design addresses problems relating to arcing and breakdown of helium in a RF plasma environment, it utilizes mechanical means of securing the insert within the pathway. For example, the insert can be held in a dielectric sleeve or secured within the pathway by a press or interference fit. Careful machining is required to provide this fit. Care must be exercised also during spray coating of the dielectric layer onto the combination of aluminum and porous dielectric surfaces as well as in forming heat transfer gas channels through the dielectric layer. Additional complications are associated with preventing undesirable cracking of the dielectric layer during the manufacture or operation of the chuck.
Therefore a need exists for an electrostatic chuck and a method of fabricating the electrostatic chuck which minimizes or overcomes the above-mentioned problems.
The present invention is related to an electrostatic chuck and to a method of fabricating an electrostatic chuck. The electrostatic chuck of the invention includes a chuck body having a back surface and a chucking surface. The electrostatic chuck also includes an electrode within said chuck body. The electrostatic chuck further includes at least one conduit for providing fluid communication between the back surface and the chucking surface. At least a portion of the conduit includes a porous region which is integrated with the chuck body.
In one embodiment of the invention the chuck body is a ceramic body. In another embodiment, the porous region has a chemical composition which is essentially the same as the chemical composition of the ceramic body. In a further embodiment, the porous region extends from the back surface to the chucking surface.
One method of fabricating an electrostatic chuck includes forming a green body of an electrostatic chuck body. The green body has at least one region which includes removable particles. The method further includes heating the green body to form the electrostatic chuck body and removing the particles, thereby forming a porous region in the electrostatic chuck body. The particles can be removed, for example, by heating or by treatment with a decomposing chemical agent.
Another method of fabricating an electrostatic chuck includes forming in a green body of an electrostatic chuck body at least one green region which, following densification of the green body, will be porous. The method also includes densifying the green body, thereby forming the ceramic electrostatic chuck body which includes at least one porous region.
In one embodiment of the invention the green region includes a sintering aid. In another embodiment of the invention the green region includes a polymeric component which during densification of the green body decomposes thereby leaving pores and forming the porous region of the electrostatic chuck body.
The invention has many advantages. For example, the electrode of the electrostatic chuck of the invention is within the chuck body and thus well isolated from the plasma environment. The porous region within the conduit for directing a heat transfer fluid to the chucking surface minimizes plasma penetration into the conduit and possible plasma extinguishing. The porous region also provides interconnecting channels and pores which are not in a straight line path thereby minimizing arcing problems. At the same time, due to the conductance of the porous region, heat transfer fluid can flow through the pores and channels in enough volume to affect the temperature of the work-piece supported onto the electrostatic chuck. Furthermore, the porous region is not held in place by mechanical means such as a sleeve or through an interference or press fitting, which require careful machining, but is an integral part of the chuck body. The invention also provides simple methods of manufacturing the electrostatic chuck of the invention.